Collegiate Church of St. Sixtus and of the Augustinian Canons St. Sebastian, Romanesque cathedral on Herrenchiemsee Island, Germany.

The Inseldom, formerly the collegiate church of St. Sixtus and St. Sebastian, is a tall, white-plastered building with a steep dark grey roof on Herrenchiemsee. Its broad facade has slightly projecting corner sections, a dark double door in a stone surround, and two vertically aligned windows above it, the upper one arched. Only the long nave survives; the towers and choir were demolished in 1818–1820.

The building served as the center of the Diocese of Chiemsee from 1215 until the diocese was dissolved in 1807 during Bavarian secularization. This event marked the end of its role as an ecclesiastical administrative center.

The Baroque frescoes and stucco work by artists from the Graubünden school still define the interior today. These decorations show the wealth and artistic vision that the church displayed during its peak as a religious center.

The former church stands on Herreninsel, reachable by boat across the Chiemsee; the interior has only been open again since 2021, and only at certain times, so check the Bavarian Palace Administration's opening days before crossing. The nave is what survives: the towers and choir were demolished in 1818–1820, so there is no tower to climb and no lift.

The brewery conversion was ordered by Munich merchant Alois von Fleckinger, who had the towers and choir demolished between 1818 and 1820 and rebuilt the nave as a brewhouse. The tall nave suited the brewhouse so well that it is the only part still standing. King Ludwig II, who bought the island in 1873, kept it running as the Kgl. Brauerei Herrenchiemsee.